The invention relates generally to control of combustion dynamics in a combustor, and more particularly to a fail-safe and tunable system and method for controlling combustion dynamics in a combustor.
Rotary engines, such as gas turbines, extract energy from a flow of combustion gas. A gas turbine typically includes a compressor coupled to a downstream turbine through a combustor. Combustion gas is generated by combusting a mixture of air and fuel in the combustor. Combustion increases the temperature, velocity and volume of the gas flow. The combustion gas is directed through a nozzle over the turbine's blades, spinning the turbine and powering the compressor.
Lean premixed combustion (LPC) is currently one of the most promising concepts for substantial reduction of emissions while maintaining high efficiency for gas turbine combustors. This mode of combustion is operated with excess air to reduce flame temperatures in combustors to acceptable levels typically less than 1800 Kelvin. At these flame temperatures, the production of thermal NOX (oxides of nitrogen) is virtually eliminated; the production of prompt NOX is negligible. This intrinsic benefit can be offset by several potential disadvantages. LPC systems can have problems with flame stability, noise, and can exhibit system dynamic responses (combustion instabilities).
Combustion dynamics (or instability) is well known problem encountered by the lean premixed combustion systems leading to operational restrictions and even to potential hardware downtime. Fluctuations in fuel-air-ratio may play a vital role in driving the combustion dynamics. There may be primarily two approaches to control combustion dynamics. The first approach may be to control the generating mechanism itself by using fuel-flow fluctuations to drive the stability, for example. A second approach may be to suppress the dynamics generated by using resonators inside combustors, for example, to suppress combustion dynamics. The approaches so far to obtain the fuel flow fluctuations have been to either use high-frequency mechanical modulator valves or fluidic oscillators. The usage of modulator valves or fluidic oscillators has a drawback that a failure of the modulator valve or fluidic oscillator can lead to a disruption of fuel supply to gas turbine causing shutdown. The characteristic response frequency and the amplitude of the fluidic oscillator cannot be controlled.
It is desirable to have a fail-safe and tunable system and method for controlling combustion dynamics in a combustor.